In electrophotographic and electrostatic printing processes (collectively electrographic processes), an electrostatic image is formed on the surface of a photoreceptive element or dielectric element, respectively. The photoreceptive element or dielectric element may be an intermediate transfer drum or belt or the substrate for the final toned image itself, as described by Schmidt, S. P. and Larson, J. R. in Handbook of Imaging Materials Diamond, A. S., Ed: Marcel Dekker: New York; Chapter 6, pp 227–252, and U.S. Pat. Nos. 4,728,983, 4,321,404, and 4,268,598.
In electrostatic printing, a latent image is typically formed by (1) placing a charge image onto a dielectric element (typically the receiving substrate) in selected areas of the element with an electrostatic writing stylus or its equivalent to form a charge image, (2) applying toner to the charge image, and (3) fixing the toned image. An example of this type of process is described in U.S. Pat. No. 5,262,259.
In electrophotographic printing, also referred to as xerography, electrophotographic technology is used to produce images on a final image receptor, such as paper, film, or the like. Electrophotographic technology is incorporated into a wide range of equipment including photocopiers, laser printers, facsimile machines, and the like.
Electrophotography typically involves the use of a reusable, light sensitive, temporary image receptor, known as a photoreceptor, in the process of producing an electrophotographic image on a final, permanent image receptor. A representative electrophotographic process involves a series of steps to produce an image on a receptor, including charging, exposure, development, transfer, fusing, and cleaning, and erasure.
In the charging step, a photoreceptor is covered with charge of a desired polarity, either negative or positive, typically with a corona or charging roller. In the exposure step, an optical system, typically a laser scanner or diode array, forms a latent image by selectively discharging the charged surface of the photoreceptor in an imagewise manner corresponding to the desired image to be formed on the final image receptor. In the development step, toner particles of the appropriate polarity are generally brought into contact with the latent image on the photoreceptor, typically using a developer electrically-biased to a potential opposite in polarity to the toner polarity. The toner particles migrate to the photoreceptor and selectively adhere to the latent image via electrostatic forces, forming a toned image on the photoreceptor.
In the transfer step, the toned image is transferred from the photoreceptor to the desired final image receptor; an intermediate transfer element is sometimes used to effect transfer of the toned image from the photoreceptor with subsequent transfer of the toned image to a final image receptor. In the fusing step, the toned image on the final image receptor is heated to soften or melt the toner particles, thereby fusing the toned image to the final receptor. An alternative fusing method involves fixing the toner to the final receptor under high pressure with or without heat. In the cleaning step, residual toner remaining on the photoreceptor is removed.
Finally, in the erasing step, the photoreceptor charge is reduced to a substantially uniformly low value by exposure to light of a particular wavelength band, thereby removing remnants of the original latent image and preparing the photoreceptor for the next imaging cycle.
Two types of toner are in widespread, commercial use: liquid toner and dry toner. The term “dry” does not mean that the dry toner is totally free of any liquid constituents, but connotes that the toner particles do not contain any significant amount of solvent, e.g., typically less than 10 weight percent solvent (generally, dry toner is as dry as is reasonably practical in terms of solvent content), and are capable of carrying a triboelectric charge. This distinguishes dry toner particles from liquid toner particles in that liquid toner particles are solvatable to some degree, typically in more than 50 weight percent of a low polarity, low dielectric carrier solvent; and liquid toner particles are generally chemically charged using polar groups that dissociate in the carrier solvent, but do not carry a triboelectric charge while solvated and/or dispersed in the liquid carrier.
A typical dry toner particle generally comprises a polymeric binder and optionally a visual enhancement additive, e.g., a colored pigment particle. The binder fulfills functions both during and after the electrophotographic process. With respect to processability, the character of the binder impacts the triboelectric charging and charge retention characteristics, flow, and fusing characteristics of the toner particles. These characteristics are important to achieve good performance during development, transfer, and fusing. After an image is formed on the final receptor, the nature of the binder (e.g. glass transition temperature, melt viscosity, molecular weight) and the fusing conditions (e.g. temperature, pressure and fuser configuration) impact durability (e.g. blocking and erasure resistance), adhesion to the receptor, gloss, and the like.
For example, polymeric materials suitable for use in dry toner particles typically have a high glass transition temperature (Tg) of at least about 50–65° C. in order to obtain good blocking resistance after fusing, yet typically require high fusing temperatures of about 200–250° C. in order to soften or melt the toner particles and thereby adequately fuse the toner to the final image receptor. High fusing temperatures are a disadvantage for dry toner because of the long warm-up time and higher energy consumption associated with high temperature fusing and because of the risk of fire associated with fusing toner to paper at temperatures approaching the autoignition temperature of paper (233° C.).
In addition, some dry toners using high Tg polymeric binders are known to exhibit undesirable partial transfer (offset) of the toned image from the final image receptor to the fuser surface at temperatures above or below the optimal fusing temperature, requiring the use of low surface energy materials in the fuser surface or the application of fuser oils to prevent offset. Alternatively, various lubricants or waxes have been physically blended into the dry toner particles during fabrication to act as release or slip agents; however, because these waxes are not chemically bonded to the polymeric binder, they may adversely affect triboelectric charging of the toner particle or may migrate from the toner particle and contaminate the photoreceptor, an intermediate transfer element, the fuser element, or other surfaces critical to the electrophotographic process. In addition to the visual enhancement additive and the polymeric binder, dry toner particles may optionally include other additives.
Charge control additives (charge directors, charge control agents or CCA's) are often used in dry toner when the other ingredients, by themselves, do not provide the desired triboelectric charging or charge retention properties. As noted above, release or slip agents may be used to help prevent the toner from sticking to fuser rolls when those are used, thereby preventing or reducing offset. Other additives include antioxidants, ultraviolet stabilizers, fungicides, bactericides, flow control agents, and the like.
Dry toner particles have been manufactured using a wide range of fabrication techniques. One widespread fabrication technique involves melt mixing the ingredients, comminuting the solid blend that results to form particles, and then classifying the resultant particles to remove fines and larger material of unwanted particle size. External additives may then be blended with the resultant particles. This approach has drawbacks. First, the approach necessitates the use of polymeric binder materials that are friable or fracturable to some degree so that comminution can be carried out. This limits the kinds of polymeric materials that can be used, including materials that are fracture resistant and highly durable. This also limits the kinds of colorants to be used, in that some materials such as metal flakes, or the like, may tend to be damaged to too large a degree by the energy encountered during comminution.
The amount of energy required by comminution itself is a drawback in terms of equipment demands and associated manufacturing expenses. Also, material usage is inefficient in that fines and larger particles are unwanted and must be screened out from the desired product. In short, significant material is wasted. Recycling of unused material is not always practical to reduce such waste inasmuch as the composition of recycled material may tend to shift from what is desired.
Relatively recently, chemically grown dry toner materials have been made using a variety of methods. In such methods, the polymeric binder is typically manufactured as a dispersion in aqueous media by solution, suspension, or emulsion polymerization techniques under conditions that form monodisperse, polymeric particles that are fairly uniform in size and shape. After the polymeric binder is formed, it is filtered and washed to remove unreacted monomer, surfactants and other extraneous material, then dried and combined with other desired ingredients to form a dry toner powder. Because the high boiling point and large latent heat of vaporization of water makes it impractical and expensive to evaporate all of the aqueous media to obtain a dry polymeric binder, drying of the binder is often effected by filtration to remove a substantial amount of the water, followed by evaporative drying to remove substantially all of the remaining aqueous media.
Solvent-based polymer dispersions in a nonaqueous liquid (organosols) have been prepared using dispersion polymerization in low polarity, low dielectric constant carrier solvents for use in making relatively low glass transition temperature (Tg≦30° C.) film-forming liquid electrophotographic toners. See, e.g., U.S. Pat. No. 5,886,067 and 6,103,781. Organosols have also been prepared for use in making intermediate glass transition temperature (Tg 30–55° C.) liquid electrostatic toners for use in electrostatic stylus printers. See e.g. U.S. Pat. No. 6,255,363 B1.
Some solvent-based polymer dispersions have also been developed for producing dry toners. See, e.g., U.S. Pat. Nos. 6,136,490 and 5,384,226 and Japanese Published Patent Document No. 05-119529. Unfortunately, the use of organosols or solvent-based polymer dispersion to make dry toner particles has proved to be substantially more challenging than the use of organosols to make liquid toner compositions. When a solvent-based dispersion is dried to remove the nonaqueous liquid carrier as is necessary to make dry toner particles, the binder particles tend to agglomerate and/or aggregate into one or more large masses. Such masses must be pulverized or otherwise comminuted in order to obtain dry toner particles of an appropriate size. The need for such comminution defeats a major advantage of using organosols in the first instance, which is the formation of substantially monodisperse, polymeric particles of uniform size and shape. In addition, it has been reported to be more difficult to incorporate slip agents (e.g. waxes) or triboelectric charge control additives (CCA's) into nonaqueous dispersions due to solubility constraints and other considerations. Consequently, the full spectrum of benefits that result from using organosols has not been realized for widespread, commercial, dry toner applications.
Particle size and charge characteristics are especially important to form high quality images with good resolution using dry toners. Dry toner particles must be as uniform in size, charge rate, and charge holding characteristics as is practically possible in order to maximize image forming performance. Accordingly, there is always a demand in this industry for techniques that yield dry toner particles with more uniform particle size, charging rate, and/or charge holding characteristics. There is also a demand for new polymeric binders for dry toners that exhibit controllable particle size, shape and charge polarity; improved charging characteristics and charge stability; improved low temperature fusing performance; and lower manufacturing cost arising from improved yields, reduced processing steps, or more efficient processing methods.